Wave length modulator



G. 1.. U SSELMAN WAVE LENGTH MODULATOR April 18, 1944.

Filed May 27,' 1942 2 Sheets-Sheet 1 l TILE .1.

INVENTOR Qsofiqe- Z551 MJIV BY M5 ATTORNEY Patented A r. is 1944 OFFICE 2.34am v WAVE IINGI'H DIODULA'I'OR George L. Ussellnan, Port .Ieiferson, N. Y., as-

porationof Delaware signor to Radio Corporation of "America, a cor- Application is, :1. 1942, Serial 110.444.4534

In this application I have disclosed anew and improved wave length modulation-system wherein frequency modulation or phase modulation is accomplished in a new andimproved manner.

' An object of the present invention is to improve wave generators and wave length modulators.

In attaining this general object I provide a new and improved frequency modulation system; in-

eluding wave generating and stabilizing means including tubes having output electrodes coupled to input electrodes bya transmission line of a length such that the circuits are regenerative. The line is.terminated by a variable impedance 7 equal to the line surge impedance. The variable impedance is modulated to produce reflection in the line to-change the phase of feed back to the tubeinput electrodes to thereby change the frequency of operation. some amplitudemodulation'may be produced and-in a modification this undesired amplitude modulation is compensated by additionally modulating the potential supplied to an electrode of one tube in'the proper sense.

Other objects and the manner in which they are attained will appear in the detaileddescription which follows.

' vIn describing my invention reference .will be 4 made to the attached drawings wherein Figs. 1

andz each illustrates the-essential features of two embodiments of my wave length modulation system. Fig. 3 is a modification of Fig. 2.

In my system, as illustrated in Fig; 1, two elec- 1 tron discharge devices VI and V2'have their anodes 8 and 8 tiedtogether and connected to a tuned tank circuit Cl-Ll. A point on this tank circuit is connected toground by a bypass condenser Ill which shunts a source of direct current potential notshown. The cathodes of tubes Vi and V2 are grounded so that they are effectively connected for radio frequency voltages to the point l2 on the inductance Li. A transmission line designated at L is connected at one terminal to a point H on line Li. Line L is shown as a concentrated reactance having distributed capacity to ground, the distributed capacity being indicated by condenser l6. Since the point it 'ls grounded 'at the generated frequency, the generated voltage between points i2 and i4 is fed to the line. The line may be a simpie two-conductor arrangement, such as, concentric conductors or may comprise wound inductto ground is insufflcient, additional capacities I 6 may be supplied. The control grids 26 and 2B are connected to two points 20 and 24 on the line and are excited by the voltages fed back over the 10 Clalms. (Cl. ire-1.71.5)

substantially opposed to the voltages on the anodes and s. In my system, the control grids between points 2| and :4 is opposed to the phase of the voltage on the anodes 6 and i so that the rid excitation voltages are sufllciently opposed to the anode voltages to insure generation of oscillations.

Then the feed-back voltage to the grids of tubes VI and V2 is in opposition to the plate voltage except for the M4 shift which occurs between said median point 35 and the points 20 and 24. With this adjustment and dimensioning of the line regeneration is provided and oscillation generation assured.

Where the line L is connected to a point on Li at which thevoltage phase is opposed to the phase of the voltage at the anodes of Vi and V2 the length of the line between the said point on Li and the median point 35 must be equal to N (M2) where N is a whole'even number equal to at least 2. Where the line L is connected to a point on line Li at which the voltage phase is substantially the same as the voltage phase at the anodes of tubes VI and V2, say point i3, then the length of the line between said point 13 on Li and the median point 35 should be N (M2) where N is a whole odd number at least equal to 1. This adjustment would be necessary when L is connected.

40 '03. The length of the line included between the points 20 and 24 should be substantially equivalent to M4. The points 20 and 24 may be moved along the line'and moved relative to each other,

provided the spacing between the two points is such that the reactance therebetween is substantially equal to M4, or an odd multiple.

The end of the line, as stated above, is shunted by an impedance and this impedance may comprise RI and the impedance between the anode and cathode of the tube V3 or the latterimance as shown. Where the distributed capacity pedance alone. The tube V3 is the modulator tube and has its control grid 30 coupled byresistance R4 to'its cathode through the secondary winding of a modulation transformer T. The re-.

sistance R4 serves to supply the bias for grid 30 and also feeds the modulation to said grid 30. Blocking condenser 34 and by-pass condenser 36 completes this modulated circuit. The primary winding of this transformer is coupled to a modulati'on source, A. The cathode of tube V3 is contank circuit and the energy therein takes up a of the feed-back energy shifts between two limits nected to ground and to the ground side of line L.

For operation, the system is adjusted as follows: The tube V3 is biased and supplied with anode direct current voltage of such a value that the desired average current therethrough is obtained under conditions of no modulation. The tube at this point may have an output impedance substantially equal to the surge impedance of the line. However, where it has not, an additional resistance BI is supplied. I now adjust the value of resistance RI until the radio frequency voltages of the generated frequency fed to both tubes Vi and V2 are of equal amplitudes. Equality of these radio frequency currents is indicated by equal currents in the ammeters AM in the grid to cathode circuits of the tubes. When this point of equal radio frequency excitation on the grids 28 and 2| is reached, the surge impedance of the line is matched by RI and the anode to cathode impedance of V3. Under these conditions, there is no reflection back along the line and no standing waves on the line. The tubes VI and V2 will now' generate oscillations of a substantially fixed frequency, which represents the carrier frequency of the transmission system.

Now if we lower the output impedance of tube V3 by say increasing the positive grid potential thereof, the line surge impedance is no longer matched. This causes reflection on the line so that standing waves are set up on the line and the radio frequency excitation on the respective tube grids changes, that is, the grid excitation of these tubes is no longer equal so that the tubes no longer feed equal amounts of current to the new phase, which is more nearly the phase of that tube feeding the greatest amount of energy to the tank circuit. In the arrangement shown the radio frequency on the grid of tube Vi drops when the impedance of tube V3 is lowered and the radio frequency excitation on the grid 28 of tube V2 goes up. When the output impedance of tube V3 is raised, the radio frequency, excitation on the grid of tube VI goes up and the radio frequency excitation on the grid of tube V2 goes down.

Since these two tubes have their grids excited in phase quadrature and since their anodes are connested in parallel to the tank circuit, the phase set by the phase displacement of the excitation voltages, and since the system is regenerative, the action is accumulative and results in frequency modulation of the output in the tank circuit.

As stated above, the points of connection between the grids of VI and V2 may be moved along the line provided the voltages picked ofl thereby are displaced by about 90. Moreover, the position of the connections may be reversed and this may be done by moving the switch 40 to the contact l2.

One of the tubes, VI or V2, may be omitted and the system will still operate to generate the frequency modulated carrier. However, in this modification amplitude modulation will be produced and more care must be taken in limiting the output.

In the system described above and illustrated in Fig. 1 some amplitude modulation may be caused due to the fact that the radio frequency voltages picked off or taken from the line and supplied to the control grids 26 and 28 may not vary equal amounts during modulation. To correct this, I supply an additional tube V4 and connect this tube in a push-pull circuit with tube V3 as shown in Fig. 2. The modulation excitation of the grid II is adjusted to the proper value to: compensation purposes. The anode of this tube is connected to the grid 2! of tube V2 by a connection at point 26 as shown and since the arrangement is a push-pull modulator, additional modulation of the 'potential on the grid 28 in accordance at V3 and V4 in Figs. 1 and 2 an auto-transformer TA maybe used to step down the tube impedance looking into line-L as illustrated in Fig. 3.

It will be understood that by modifying the modulating potentials in a well-known manner phase modulation may be accomplished in the v system.

Iclaim:

1. In a wave length modulation system, a pair of electron discharge devices each having an anode, a cathode, and a control grid, a tuned tank circuit, connections tying said anodes together and to one end of said tank circuit, a coupling between the cathode of said tubes and a point on said tank circuit, a two-wire transmission line comprising reactance including distributed capacity, a coupling between one end of said line and the reactance of said tank circuit, a variable impedance in shunt to the other end of said line, said variable impedance substantially matching the surge impedance of said line, connections coupling the control grids of said tubes and spaced points on said line to feed voltages to said control grids which substantially oppose the voltages on the anodes of said tubes whereby oscillations are generated in said tubes and set up in said tank circuit, the distance between said points being substantially equal A/4,-where i. is the length of the oscillations generated, and means for controlling said variable impedance in accordance with signals.

2. In a wave length modulation system, a pair of electron discharge devices each having an anode, a cathode, and a control electrode, a tank circuit including an inductance, connections tying the anodes of said tubes together and to a point on said tank circuit, connections coupling the cathodes of said tubes together and to a second point on said tank circuit, a transmission line having a length equal to -N( ./2) ./8 where N is a whoienumber at least equal to 1, a connection between one end of said line and the inductance of said tank circuit, a surge matching impedance connected to the other end of said line, said surge matching impedance including the impedance be-' tween the output electrodes of an additional tube, connections coupling the control electrodes of said devices to points on said line spaced a distance substantially equal to A/4, means for adjusting the impedance at the said other end of said line to a value at which it matches the surge impedance of :aid line, and means for modulating the impe ance 0 said additional tube in acco d with signals. r 3. In a wave length modulation system, an

I electron tube generator having a control electrode and having a tank circuit coupled to its anode; a transmission line having one end coupled to said tank circuit and the other end coupled to the control electrode of said tube to feed thereto a voltage the phase of which is substantially opposed to the phase of the generated voltage on the anode of said tube, a variable impedance substantially equal to the surge impedance of the line of the voltage fed to said control electrode of said tube and means for compensating the effect of said variations in the amplitude of said voltage fed to the control electrode of said tube.

4. Ina wave length modulation system, a pair of electron discharge devices each having an anode, a cathode, and a control grid, a tuned tank circuit, connections tying said anodes together and to one end oi! said tank circuit, a coupling between the cathode 01' said tubes and a point on said tank circuit, a two-wire transmission line comprising reactance including distributed capacity, a coupling between one end of said line and the'reactanceoi said tank circuit, a variable impedance in shunt to the other end of said line, said variable impedance matching the surge impedance of said line, connections coupling the control grids of said tubes to points on said line, the distance between which is substantially equal to M4, means for controlling said variable impedance in accordance with signals, and means for compeneating for undesired amplitude variation of the vvoltages in said line caused by variation oi said impedance.

5. In a wave length modulation system, a pair of electron discharge devices each having an anode, a cathode, and a control grid, a tank circuit including an inductance, connections tying,

said tubes and tank circuit, means for adjusting the impedance at the said other end of said line to a value at which it matches the surge impedance of said line and means for modulating the impedance of said device in accordance with said modulating potentials to correspondingly modulate the impedance at the said other end of the line and correspondingly modulate the length of the waves generated.

including the impedance between the output electrodes of an additional tube, connections cou+ pling the control devices of said devices to points' on said lines spaced a distance substantially equal to we, means for adjusting the impedance at the end of said line to a value at which it'matches the surge impedance of said line, means for modulatthe impedance of said additional tube in accordance with signals, a modulation compenea tube having output electrodes coupled to one ofsaid first named devices, and means for modulating the impedance of said-second tube in accordance with said signals.

6. 'a ailing system, wave generating me including a pair of electron discharge tubes each having an anode, a cathode, and a control grid,'a tank circuit including an inducce, connections tying the anodes of said tubes together and to a point on said tank circuit, connections coupling the cathodes of said tubes to-. ather andto a point on said tank circuit, a trans inclu the impedance between the output electrodes of an additional electron discharge device, connections connecting the control grids of said tub to points on said lines spaced a distance it 1 r isgenerated in 7,. In a wave length modulation system, an electron tube generator having a control electrode and having a tank circuit coupled to its anode, a transmission line having a length about equal to N (M2) where N is a whole number at least equal to l, a coupling between one end of said line and said tank circuit; a coupling between the 'other end of said line and the control electrode of said'tube to feed thereto generated voltage the phase of which is substantially opposed to the phase of the generated voltage on the anode of said tube whereby wave energy is generated in said tube, a variable impedance substantially equal to the surge impedance of the line shunting said other end of said line, connections for controlling said variable impedance in accordance with signals to correspondingly vary said phase relation of the voltages on the control electrode and the anode of the tube to thereby vary the length of the wave energy generated, and connections for compensating undesired amplitude modulation of the wave energy caused by variations of the amplitude of the voltage fed to said control grid due tocontrolling said impedance in accordance with signals.

.8. In a wave length modulation system, a pair of electron discharge devices each having an anode, a cathode, and a control electrode, a tank circuit, connections tying the anodes of said tubes together and coupling them to a first voltage point "on said tank circuit, connections coupling the cathodes of said tubes together and to a second voltage point on said tank circuit, a transmission line having a length equal to N (A/2)+ \/8 where N is a whole number at least equal to 1, a coupling between one end of said line and said tank circuit, a surge matching impedance connected to the other end of said line, said surge matching impedance including the impedance between electrodes of an additional tube, connections coupling the control electrodes of said devices to points on said transmission line spaced apart 'a distance substantially equal to )i/d,

means for adjusting the impedance at the said other end of said transmission line to a value at which it matches the surge impedance of said line, and means for modulating the impedance oi said additional tube in accordance with signals.

9. A system as recited in claim 8 wherein N is an odd number and said coupling between said one end of said line and said tank circuit and said point on'said tank circuit to which the anodes are coupled are on the same side of the said second point on said tank circuit to which the cathodes are coupled.

10. A system as recited in claim 8 wherein N is an even number and said coupling between said one end of said line and said tank circuit and said point on said tank circuit to which the anodes are coupled are onfopposite sides 0! the said second point on said tank circuit to which the cathodes are coupled. 

